The glass/metal joins are currently used in a wide variety of technological fields such as the electronic, electrical and medical industry, etc. These joins are especially useful for devices that have to operate under vacuum conditions, because the joins formed between the glass and the metal are quite strong and hermetical. The properties of these glass/metal joins lie in the good structural characteristics of the materials forming them, their high insulating capacity, their chemical inalterability and their impermeability to solids, liquids and gases.
These types of joins have been developed recently for solar collectors tubes parts, which requires durability over time supporting inclement weather (changes in temperature between day and night, between seasons), mechanical and thermal stresses, working temperatures of up to 400° C. and full sealing.
There are several types of glass/metal joins:
    1. Joins in which the thermal expansion coefficients match: Joins where the coefficient of thermal expansion and contraction of the metal and glass are similar, such that the resulting joins have tensions within a safe limit, both during the elaboration of the join and throughout the life span of the device.    2. Joins in which the thermal expansion coefficients do not match: Joins where the coefficient of thermal expansion and contraction of the metal and glass differ considerably, so that the tensile stresses resulting from this join are ameliorated by using:            Small diameter metals, giving rise to the so-called compression joins.        Ductile metals that dampen part of the tensile stresses generated.        Intermediate glasses, such that the thermal expansion coefficients adjust such that the last join between the glass and the metal will be of the first type.            3. Welding joins, in which the metal part is welded to a metal layer that has been previously deposited at the surface of the glass by any of the different existing methods.    4. Mechanical joins between the glass and the metal.
Currently the majority of the glass/metal joins are obtained by the first two methods.
The most important conditions for producing the glass/metal join are the following: that there is a good adherence between the materials to be joined, that the thermal expansion coefficients are compatible and that the softening temperature of the used glass is relatively low and that this working temperature does not affect the material to be joined.
One of the key elements of the solar power plants based on parabolic cylinder collectors (PCC) is the absorber tube and although they are currently being developed and improving the functioning of said solar power plants such that energy is generated in a more efficient and respectful with the environment manner, due to the high cost of the devices that make it up, especially the absorber tubes, the life span should be extended as much as possible (10-20 years).
The absorber tube consists of two tubes arranged in a concentric way, a glass tube arranged in the outer part and another metal tube placed in the inner part, through which a liquid circulates, usually synthetic oil, which heats up by the effect of the solar rays up to approximately 400° C. This liquid after a series of processes produces a superheated steam which is finally transformed into electrical energy.
The two tubes that form part of the absorber tube are separated by a vacuum chamber and, in turn, they are joined through a glass/metal join. Due to the large dimensions and weight of the absorber tube (approximately 4 meters in length and 100 mm diameter), together with the vitreous nature of the outer tube and since these devices are placed in the open air and they are subject to experience the inclement weather (hail, rain and erosion from particles of dust, etc.) make the glass/metal join one of the most important and weak parts of the parabolic cylinder collector. Moreover the sealing between the two tubes must be hermetical and the vacuum kept such that the heat loss is minimized and the power generation is optimized.
The first attempts to obtain this type of glass/metal joins present in the solar collectors were made with an approximation of the HouseKeeper method where a thin layer of copper and a glass, the thermal expansion coefficients of which differed considerably, was used. The metal was thinned to facilitate the formation of the glass/metal join and subsequently the glass was heated such that the metal could be introduced inside. Due to the small thickness of the metal and to the ductile nature of copper the tensile stresses generated due to the differences between the thermal expansion coefficients were minimized and a satisfactory glass/metal join was obtained.
Due to the nature and the thickness of the used metal, this was able to withstand the tensile stresses generated during the cooling of the piece as well as during the life span of the device. However, this type of glass/metal welding did not completely meet the requirements of these systems since they were the weakest part of said system and therefore one of the parts to be improved.
Subsequently a new step forward for this kind of systems was introduced, such as the one consisting of using a metal and a glass, the thermal expansion coefficients of which are very similar such that the join is stronger than the one achieved through the HouseKeeper method and therefore the duration of the system will also be greater.
Currently at the commercial level a glass and a metal alloy are used the thermal expansion coefficients of which are around 5×10−6° C.−1.
The glasses have many qualities that make them appropriate in the vacuum industry and these advantages are, among others:                They are virtually impermeable to gases.        According to their chemical composition they can selectively transmit at several wavelengths.        They have good electrical insulation properties.        They are easily molded over a flame.        By choosing a particular chemical composition the properties of the glass can be varied such that glasses with properties that match those of the desired metal can be obtained.        
It is in this line that the invention proposed in the following is developed.